1. Field of the Invention
This invention relates generally to X-ray tubes and is concerned more particularly with a rotatable X-ray target having improved heat emissive surface means.
2. Discussion of the Prior Art
Generally, an X-ray tube of the rotating anode type includes an evacuated envelope having therein an electron emitting cathode disposed to beam high energy electrons onto an aligned focal spot area of a spaced anode target. The target may comprise a rotatable disc having adjacent its outer periphery an annular focal track made of X-ray emissive material, such as tungsten-rhenium alloy, for example. Thus, electrons beamed from the cathode may be focused onto a focal spot surface area of the focal track to penetrate into the underlying material and generate X-rays which radiate therefrom and out of the tube.
Most of the electron energy incident on the focal spot area of the focal track is converted to heat energy which could become excessive and damage the surface of the focal track. Consequently, the target disc usually is mounted for axial rotation by having a central portion thereof attached to a stem end portion of a magnetic induction rotor which is supported by bearings in the envelope. Thus, an external stator encircling the rotor may be adjustably energized for rotating the rotor and attached disc to move successive radial segments of the focal track at a desired speed through the focal spot area aligned with the electron beam. As a result, heat energy is accumulated in the body of the target and must be dissipated therefrom before becoming sufficiently excessive to cause warping or cracking of the disc.
However, the stem end portion of the rotor generally is provided with a suitable cross-sectional configuration for restricting the flow of heat from the attached disc to the rotor in order to prevent damage to the support bearings thereof. Therefore, the heat energy accumulated in the target disc cannot be dissipated efficiently by conduction through adjacent structure, and cannot be dissipated by convection since the tube envelope is evacuated. Consequently, the accumulated heat energy in the target disc is dissipated predominantly by radiation to the tube envelope, which may be cooled by a dielectric fluid, such as an X-ray transparent oil, for example.
Thus, it is advantageous and desirable to provide an X-ray tube with a target having improved heat emissive surface means for dissipating heat from the target.